Ion-focused electron beam directional magnetometer

ABSTRACT

Described is an electron beam magnetometer of the type adapted to detect disturbances in magnetic fields, particularly the magnetic field of the earth. This is accomplished by means of a space-charge-neutralized low voltage electron beam. Positive ions, obtained either by ion injection or weak ionization of a low pressure gas within an envelope through which the electron beam passes, hold the beam together along its path. The beam is normally centered by means of conventional horizontal and vertical deflection coils. Disturbances in a magnetic field traversing the beam cause it to deflect; and this deflection is sensed to indicate the disturbance.

' United States Patent I Goldman et al.

[15] 3,657,642 1 Apr. 18, 1972 [54] ION-FOCUSED ELECTRON BEAM 2,666,268 1/1954 Kliever ..324/44 ux DIRECTIONAL MAGNETOMETER OTHER PUBLICATIONS [72] Inventors: Michael A. Goldman, Pikesville; Harry Goldie, R an danst ow n, both M d. Kliever et al., Product Engmeermg, Aug. 1947, pgs. 132- 134. [73] Assignee: Westinghouse Electric Corporation, Pitt- Primary Examiner-Alfred E. Smith sburgh, Pa. Att0rney-F. H. Henson [2]] APPI'NOJ 862278 Described is an electron beam magnetometer of the type 6 adapted to detect disturbances in magnetic fields, particularly [52] US. Cl.... ..324/44 the magnetic field Ofihe earth. This is accomplished by means [51] In C] Gob. 33/02 of a space-charge-neutralized low voltage electron beam. 58 Field of Search ..324/44 43 41- 313/73 Obtained either by ion injection 0r Weak ionization of a low pressure gas within an envelope through which the electron beam passes, hold the beam together along its [56] References cued path. The beam is normally centered by means of conven- UNITED STATES PATENTS tional horizontal and vertical deflection coils. Disturbances in a magnetic field traversing the beam cause it to deflect; and- 2,719,248 9/1955 Josephson et al ..3 13/73 X this deflection is sensed to indicate the disturbance 2,164,302 7/1939 Barnes etal. ..324/37 l,836,569 12/1931 Benjamin ..3 13/72 7 Claims, 3 Drawing Figures Z IND/CA r/n/a l0 DEFLEC770N cv/L 7; T CIRCUIT CONT/POL PATENTEDAPR 18 m2 lNDlCAT/NG OEFLECT/ON CO/L CONTROL FIG. 2.

a M M MD m N6 I A a M C I u HARRY GOLOIE ION-FOCUSED ELECTRON BEAM DIRECTIONAL MAGNETOMETER CROSS-REFERENCES TO RELATED APPLICATIONS This application is related to application Ser. Nos. 862,376

and 862,375 both of which were filed of even date with the present application and assigned to the same assignee.

BACKGROUND OF THE INVENTION In the past, magnetometers have been devised for detecting disturbances in the earths magnetic field on the principle that an electron beam, which is centered under the influence of a normal magnetic field, will be deflected whenever that field is disturbed. This deflection, in turn, can be sensed to actuate an alarm or can be used to drive a meter or recorder which indicates' the magnitude of the deflection,

I Ordinarily, the electron beam of such magnetometers is caused to pass through an evacuated envelope to a target in much the same way as the electron beam of a conventional cathode ray tube. On either side of the evacuated envelope or within the envelope itself are pole pieces between which the electron beam passes. When a magnetically permeable object intersects the magnetic field to which the detecting apparatus is subjected, the field existing across the aforesaid gap between the pole pieces is altered. This alteration, in turn,

. causes a deflection in the electron beam which can be'used to SUMMARY OF THE INVENTION As an overall object, the present invention seeks to provide a riew and improved magnetometer which is highly sensitive to changes in a surrounding magnetic field.

More specifically, an object of the invention is to provide a magnetometer wherein an electron beam is caused to traverse an enclosure containing a gas at low pressure in order to produce positive ions. The positive ions negate the normal space-charge repulsion effects within the beam and allow it to remain collimated when using a low beam voltage over a relatively large distance.

Still another object of the invention is to provide a magnetometer in which a low voltage electron beam is caused to traversea relatively long distance, thereby increasing the sensitivity of the device to small disturbances in a magnetic field .to which it is subjected.

In accordance with the invention, apparatus for detecting disturbances in a magnetic field is provided comprising an envelope containing an ionizable gas, together with an electron emitting gun disposed within the envelope whereby an electron beam emitted bythe gun will pass through the gas and ionize the same. In the ionization process, positive ions are formed which negate the space-charge repulsion effects of the electrons within the beam and allow it to remain collimated when using low beam voltages over a relatively long distance.

At least a portion of the envelope is formed from non-magnetically permeable material whereby the electron beam, when subjected to an external magnetic field, will be deflected, The electron beam is normally directed against a split target which is adapted to produce an electrical signal indicating deflection of the electron beam.

A split target preferably is comprised of an inner section formed from four segments of a circle and an outer surrounding annular portion, also formed in segments. The outer portion is used for static positioning of the beam; whilethe inner portion is used, to detect deflection due to a disturbance in the magnetic field to which the beam is subjected.

The above and other objects and features of the invention will become apparent from the following detailed description taken in connection with the accompanying drawings which form a part of this specification, and in which:

1 is a schematic diagram of the magnetometer of the invention;

FIG, 2 is a cross-sectional view of one embodiment of the magnetometer of the invention; and

FIG. 3 is a cross-sectional view taken along line IIIIII of FIG. 2 showing the configuration of the split target of the invention.

With reference now to the drawings, and particularly to FIG. 1, an electron gun is shown comprising an electronemitting cathode 10, one or more grids l2 and an anode 14, all in accordance with the usual practice. The electron beam 16 emitted by the electron gun is caused to pass through a gaseous atmosphere of an ionizable gas to a split target configuration 18, best shown in FIGS. 1 and 3. It includes an inner s'egmented annular portion 20 formed from segments V through V, and an outer surrounding annular portion 22 formed from segments V through V,,. The outer segments V through V,, are all connected to a deflection coil control circuit 24 which, in turn, is connected to vertical deflection coils 26 and 28 and horizontal deflection coils 30 and 32, respectively. The inner set of segments V through V are connected to an indicating circuit, generally indicated by the reference numeral 34.

The mechanical details of the magnetometer of the present invention are shown in FIG. 2. It comprises a glass envelope 36 having an electrostatically shielded electron gun 38 atone end thereof, this gun including the elements 10, 12 and 14 of FIG. I. The gun 38 is provided as shown with prongs 40 which project through the glass envelope 36 for connection to external circuitry, not shown. Surrounding the glass envelope 36 in the area of the electron gun 38 is a cylindrical shield 42 of magnetically permeable material which prevents an external magnetic field from influencing the operation of the gun 38. The electron beam 16 emitted by the gun 38 passes through an aperture 44 in a wall 46, also preferably formed from magneti cally permeable material. The beam, however, after it passes through the aperture 44, is subjected to an external magnetic field, such as the earths magnetic field, indicated by the arrows 48 in FIGS. 1 and 2. The electron beam 16 is then directed against the split target comprising the inner and outer segmented portions 20 and 22.

The entire glass envelope 36 is filled with an ionizable gas, such as neon or other noble gas or hydrogen at a pressure preferably of about 0.001 to 0.20 torr. Thus, as the electron beam traverses the space between the gun 38 and the target at the other end of the glass envelope 36, it will ionize the gas immediately around it. In this process, positive gas ions are generated around the beam which attact the electrons and negate the space-charge repulsion effects within the beam itself. That is, due to the presence of the positive ions, the electrons, which would otherwise repel each other, are more or less neutralized. This causes the beam to be collimated as it travels from the gun 38 to the target at the other end of the envelope 36. Furthermore, by virtue of the fact that a very low beam voltage can be employed due to the collimation effect mentioned above, very small disturbances in the magnetic field 48 will cause the beam 16 to become deflected.

The use of this soft, low voltage beam in conjunction with increased length of the beam produces a fundamental improvement in magnetometer sensitivity in accordance with the relation for small deflection:

X= l4.8BL /(E)" where X is the lateral deflection; L is the length of the beam subjected to the magnetic field; E is the beam voltage; and B is the magnetic field in gauss.

With reference again to FIG. 1, the voltages on the outer segments V through V,, are applied, via resistors R through R to the deflection coil control circuit 24. A delay circuit 23 is interposed between segments V through V, and the deflection coil control circuit 24. The design of thedelay circuit would be a simple matter for one skilled in the art. The presence of a delay is required in the positioning circuits, so that the indicated disturbance can be recorded before the beam is repositioned to center.

The outer segments V through V,, are employed to initially center the beam with respect to the target 18 under the normal influence of the magnetic field 48. That is, if the beam should move to the right in FIG. 1, for example, the negative charge on sector V, would become greater than that on sector V,,. This increased charge, when applied to the deflection coil control circuit 24, will cause the horizontal deflection coils 30 and 32 to again center the beam 16. Similarly, if the beam moves upwardly such that the negative charge on sector V,, is greater than that on sector V,,, the unbalanced charge on sectors V,, and V when applied to the control circuit 24, will actuate the vertical deflection coils 26 and 28 to move the beam downwardly and again center it.

The inner sectors V through V, operate in the same manner. However, they are used only to indicate an unbalanced charge pattern on the sectors and, hence, deflection of the beam. If it is assumed, for example, that the magnetic field 48 is disturbed, variation in the magnetic field will cause the beam to deflect; and this will be immediately indicated by the indicating circuit 34. At the same time, the charge on the outer sectors V,, through V,, will become unbalanced, however the beam will not be recentered until after a predetermined time delay because of the built-in delay in the positioning circuits. Hence, the inner group of sectors V through V immediately indicates the disturbance in the magnetic field while the outer group of segments V,, through V, recenters the beam after a predetermined time delay which is sufficient to record the disturbance prior to beam recentering.

A brief computation for a practical beam of 25 volts over an interaction space of one foot shows that a field of a milligauss deflects the beam 1 l milliinches. However, the sensitivity depends primarily on the ability to detect the change in the number of electrons collected by the sectoral plates in order to develop a change in potential pairs.

Although the invention has been shown in connection with a certain specific embodiment, it will be readily apparent to those skilled in the art that various changes in form and arrangement of parts may be made to suit requirements without departing from the spirit and scope of the invention.

We claim as our invention:

1. In apparatus for detecting disturbances in a magnetic field caused by imposition of a magnetically permeable member in said field, the combination of an envelope containing an ionizable gas, an electron emitting gun disposed within said envelope such that an electron beam emitted by said gun will pass through said gas and ionize the same, at least a portion of said envelope being formed from non-magnetically permeable material whereby said electron beam will be deflected when subjected to an external magnetic field, and a split target against which said electron beam is directed and adapted to produce an electrical signal indicating deflection of said electron beam, the pressure of said gas and the beam voltage of said electron beam being such to produce positive ions which negate the usual space-charge repulsion effects within the beam whereby it will remain collimated along its length between said gun and said target.

2. The apparatus of claim 1 including a magnetic shield surrounding said electron emitting gun.

3. The apparatus of claim 1 including electron beam deflection devices positioned on either side of said beam, and means connecting said split target to said electron beam deflection devices whereby the beam will be recentered after it is deflected by a disturbance in said magnetic field.

4. The apparatus of claim 3 wherein said split target is formed from inner and outer annular parts each formed from segments which are electrically insulated from each other, only the outer set of segments being connected to said beam deflection devices.

5. The apparatus of claim 4 including indicating circuit means, and means connecting only said inner segments to said indicating circuit means whereby deflection of said beam due to a disturbance in a magnetic field to which it is subjected will unbalance the charges on said inner segments to actuate said indicating circuit means.

6. The apparatus of claim 5 wherein said outer segments are connected to said beam deflection devices through time delay means, whereby the beam will not be immediately recentered by the beam deflection devices after it is deflected by a disturbance in said magnetic field.

7. The apparatus of claim 6 wherein said beam deflection devices comprise electromagnetic coils. 

1. In apparatus for detecting disturbances in a magnetic field caused by imposition of a magnetically permeable member in said field, the combination of an envelope containing an ionizable gas, an electron emitting gun disposed within said envelope such that an electron beam emitted by said gun will pass through said gas and ionize the same, at least a portion of said envelope being formed from non-magnetically permeable material whereby said electron beam will be deflected when subjected to an external magnetic field, and a split target against which said electron beam is directed and adapted to produce an electrical signal indicating deflection of said electron beam, the pressure of said gas and the beam voltage of said electron beam being such to produce positive ions which negate the usual space-charge repulsion effects within the beam whereby it will remain collimated along its length between said gun and said target.
 2. The apparatus of claim 1 including a magnetic shield surrounding said electron emitting gun.
 3. The apparatus of claim 1 including electron beam deflection devices positioned on either side of said beam, and means connecting said split target to said electron beam deflection devices whereby the beam will be recentered after it is deflected by a disturbance in said magnetic field.
 4. The apparatus of claim 3 wherein said split target is formed from inner and outer annular parts each formed from segments which are electrically insulated from each other, only the outer set of segments being connected to said beam deflection devices.
 5. The apparatus of claim 4 including indicating circuit means, and means connecting only said inner segments to said indicating circuit means whereby deflection of said beam due to a disturbance in a magnetic field to which it is subjected will unbalance the charges on said inner segments to actuate said indicating circuit means.
 6. The apparatus of claim 5 wherein said outer segments are connected to said beam deflection devices through time delay means, whereby the beam will not be immediately recentered by the beam deflection devices after it is deflected by a disturbance in said magnetic field.
 7. The apparatus of claim 6 wherein said beam deflection devices comprise electromagnetic coils. 